There are many contexts in which there is a need for containers of plastics materials where the shape of the container does not alter at elevated temperature. This requirement is particularly pressing in applications where the containers are filled with pressurized contents, e.g. carbonated drinks, since the increased internal pressure at elevated temperature contributes towards deforming the shape of the container.
In the case of containers of polyethylene terephthalate, often abbreviated as PET, care is taken to ensure that the container consists of oriented material. Such material is characterised in that after stretching it shrinks in the direction of stretching when the temperature of the material is raised. An attempt has been made to remedy this problem by temperature-stabilizing the material, which implies that the material in the moulded container is thermo-crystalized by heating to a temperature above the glass transition temperature of the material. This also releases the internal stresses in the oriented material which were incorporated into it in conjunction with its stretching.
Packs, e.g. for beverages, are manufactured in very large quantities, for which reason a reduction of the quantity of material in every individual pack contributes towards lowering the total costs. It is also desirable from the point of view of the consumer that the price that is paid for a purchased item relates to as large as possible an extent to costs for the product and to as small an extent as possible to costs for the packaging. Thus both the consumer and the manufacturer have strong reasons for wishing to keep the costs for the packaging as low as possible.
In order to reduce the quantity of material in bottles of PET by as much as possible, one chooses to allow the material in at least the container body to consist of biaxially oriented material. Cf e.g. U.S. Pat. No. 3,733,309.
In recent times new technology has also been presented, refer e.g. to the British patent application GB No. 2 092 943 in accordance with which a preform consisting of material stretched in an axial direction and oriented is formed from a primarily amorphous blank in a separate processing stage. In a subsequent processing stage the orifice section of the preform is usually provided with a shape coresponding to the shape of the orifice section of the future container, after which the preform is placed in a blow mould and expanded to abutment against the walls of the blow mould in order to form the container. Cf the British patent application GB No. 2 076 731. The axially stretched material possesses improved strength characteristics, which enables reduction of the quantity of material in the neck and orifice sections of the container. In conjunctin with the pretreatment the axially oriented material in the preform is also temperature-stabilized as a rule. A container which is produced in accordance with the technology described in this paragraph thus consists of a lesser quantity of material for a certain inner space than a container which is moulded from an amorphous blank with unchanged material thickness in the orifice and neck sections.
It is known from e.g. U.S. Pat. No. 4,264,558 of a temperature-stabilized container, where the container body consists of biaxially oriented material and the orifice and neck sections of the container consist of thermo-crystallized material, whereby the thermocrystallization is allowed to proceed as along as the material in the neck and orifice sections is opaque. The thermocrystallization in accordance with the technology described in this patent requires relatively long processing times. Moreover, it is so that the container described in the patent contains a greater quantity of material than containers for an equivalent volume that are produced in accordance with the technology described in the preceding paragraph.